A foreshock is an earthquake that occurs before a larger seismic event (the mainshock) and is related to it in both time and space. The designation of an earthquake as foreshock, mainshock or aftershock is only possible after the full sequence of events has happened.[1]


Foreshock activity has been detected for about 40% of all moderate to large earthquakes,[2] and about 70% for events of M>7.0.[3] They occur from a matter of minutes to days or even longer before the main shock; for example, the 2002 Sumatra earthquake is regarded as a foreshock of the 2004 Indian Ocean earthquake with a delay of more than two years between the two events.[4]

Some great earthquakes (M>8.0) show no foreshock activity at all, such as the M8.6 1950 India–China earthquake.[3]

The increase in foreshock activity is difficult to quantify for individual earthquakes but becomes apparent when combining the results of many different events. From such combined observations, the increase before the mainshock is observed to be of inverse power law type. This may either indicate that foreshocks cause stress changes resulting in the mainshock or that the increase is related to a general increase in stress in the region.[5]

Other Languages
العربية: هزة مستبقة
azərbaycanca: Foreşok
čeština: Předtřes
فارسی: پیش‌لرزه
한국어: 전진 (지진)
Bahasa Indonesia: Gempa awal
magyar: Előrengés
Bahasa Melayu: Kejutan depan
日本語: 前震
norsk: Forskjelv
română: Preșoc
русский: Форшок
Simple English: Foreshock
српски / srpski: Форшок
українська: Форшоки
Tiếng Việt: Tiền chấn
中文: 前震